High temperature transducer using SOI electronics

ABSTRACT

There is disclosed a high temperature pressure sensing system which includes a SOI Wheatstone bridge including piezoresistors. The bridge provides an output which is applied to an analog to digital converter also fabricated using SOI technology. The output of the analog to digital converter is applied to microprocessor, which microprocessor processes the data or output of the bridge to produce a digital output indicative of bridge value. The microprocessor also receives an output from another analog to digital converter indicative of the temperature of the bridge as monitored by a span resistor coupled to the bridge. The microprocessor has a separate memory coupled thereto which is also fabricated from SOI technology and which memory stores various data indicative of the microprocessor also enabling the microprocessor test and system test to be performed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/291,868, entitled “High Temperature Transducer Using SOIElectronics,” filed Nov. 14, 2008, now U.S. Pat. No. 7,861,597, which ishereby incorporated by reference as being set forth in its entiretyherein.

FIELD OF THE INVENTION

This invention relates to pressure transducers and more particularly toa pressure transducer using a SOI sensor and SOI Electronics.

BACKGROUND OF THE INVENTION

The use of silicon-on-insulation (SOI) architecture for high temperatureoperation of a pressure transducer is depicted in U.S. Pat. No.7,231,828 entitled High Temperature Pressure Sensing System issued onJun. 14, 2007 to A. D. Kurtz et al and assigned to Kulite SemiconductorProducts Inc., the assignee herein. In that patent, there is disclosed ahigh temperature pressure sensing system including a pressuretransducer. The system includes a pressure sensing piezoresistive sensorformed by a silicon-on-insulator (SOI) process. There is a SOI amplifiercircuit which is coupled to the piezoresistive sensor, a SOI gaincontroller circuit, including a plurality of resistances that whenselectively coupled to the amplifier, adjusts the gain. There is aplurality of off chip contacts corresponding to the resistorsrespectively, for electrically activating the corresponding resistorsand using a metallization layer for the SOI sensor and SOI ASIC suitablefor high temperature interconnections wherein the piezoresistive sensoramplifier circuit and gain control circuit are suitable for use inenvironments having a temperature greater than 175° C. and reachingbetween 250° C. and 300° C. Thus in the above noted patent namely U.S.Pat. No. 7,231,828 there is a described a SOI or silicon-on-insulatorstructure which employs a SOI pressure sensor device.

As is clear from the above-noted patent, by utilizing such devices, onecan achieve high temperature operation and for example, operation intemperature ranges greater than 175° C. The above-noted patent describesvarious techniques utilized for providing SOI CMOS structures where thecomponents are N-channel and P-channel transistors, diodes, capacitorsand so on. Basically the fabrication process consists of producing athin, single crystalline layer of silicon separated from the substratevia a high quality silicon dioxide or SiO₂ layer. This can be done byusing an oxygen implantation approach where the implanted oxygen createsan insulating silicon dioxide layer some distance from the top surfaceestablishing a thin silicon layer isolated from the substrate. This canalso be produced by fusion bonding an oxidized substrate wafer to asecond wafer followed by selective etching of the second wafer to leaveonly a thin, high quality, layer of silicon over the SiO₂ layer on thesubstrate.

The process for the selective etching of the second silicon wafer canuse either a conductivity selective etching process or a lap and polishprocess or a hydrogen implant and micro-splitting process. Once thesilicon-on-insulator wafers are produced, selective doping andpatterning, additional film growing and other semiconductor processingcan be used to fabricate different features and components in thedevice. With controlled doping, the appropriate drain and source regionsin the respective transistors are provided. A high quality oxide layeris then grown to serve as a gate oxide, over which a poly-crystallineP-type silicon will be deposited to act as a gate material. The samepoly-crystalline material can also be used to form resistors as well asother components utilized in the chip. In this process, unlike bipolartechnology where NI-CR having very low thermal temperature coefficientof resistance typically are used, resistors can not be made as metallayers. As a result, the utilized poly-crystalline resistors have arelatively large TCR of about 1500 ppm/0° C. orders of magnitude higherthan that of the metal film resistors. With the SOI approach, allassociated components are dielectrically isolated from each other andfrom the substrate, thus eliminating the effects of leakage currents andsubstrate parasitic capacitance. The use of SOI enables the fabricationof very stable devices operational up to and above 300° C. and suitablefor high voltage and low voltage applications. This technology has beenexplained in detail in the above-noted patent, namely U.S. Pat. No.7,231,828 which is incorporated herein in its entirety. In any event,that patent describes in detail the advantages of SOI technology inproviding sensor devices.

It is an object of the present invention to provide circuitry which addssignificant capabilities to the operation of pressure transducers whilemaintaining high temperature operations and so on. As indicated above,the above-noted patent, namely U.S. Pat. No. 7,231,828 depicts theimplementation and advantages gained by using a SOI sensor with SOIelectronics. In this manner, one can implement a transducer with a highlevel output capable of operating at temperatures in excess of 250° C.and potentially up to 300° C. The circuit to be described uses analogcomponents as well as digital components. The analog components, forexample, may be operational amplifiers and regulators. Thus, as will beshown, the present invention includes further enhancements of theelectronic circuit with significant advantages. The enhancementsdisclosed here are digital circuits which add significant capabilitiesto the operation of the transducer, while maintaining the capability ofoperation at high temperatures of +250° C. and even up to 300° C. Thisfeature is possible due to the dielectric isolation of each device inthe SOI chip which eliminates leakage currents. The leakage currents arethe major factor limiting the operation of circuits using conventionaltechnologies.

BRIEF SUMMARY OF THE INVENTION

A high temperature pressure sensing system, comprising: a pressuresensing piezoresistive sensor formed by a silicon-on-insulator (SOI)process, an analog to digital converter formed by a silicon-on-insulator(SOI) process having an input coupled to said sensor and having anoutput providing a digital signal proportional to a sensor output, amicroprocessor formed by a silicon-on-insulator process (SOI) and havinga first input coupled to said output of said SOI analog to digitalconverter for providing at an output a digital signal indicative of thesensor output, a SOI memory coupled to said microprocessor and operativeto provide at least a portion of the control program for saidmicroprocessor.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a block diagram of a high temperature transducer using SOIsensor and SOI electronics according to this invention.

FIG. 2 is a block diagram depicting the memory shown in FIG. 1 anddepicting the various functions the memory can accommodate.

DETAILED DESCRIPTION OF THE INVENTION

Thus, referring to FIG. 1, there is shown a microprocessor 30, an analogto digital converter 19, an analog to digital converter 20 and a memory31 which is coupled to the microprocessor. Using these circuits adigital transducer is implemented, which transducer via its digitaloutput 35 is capable of communicating with digital systems. Themicroprocessor 30 allows digital compensation of the transducer as willbe explained. This achieves a much better accuracy than by employingsimilar analog techniques. The memory circuit will allow the storage ofother useful information, like the part number of the transducer, theserial number, the last calibration date as well as the capability ofre-calibration of the transducer without opening its hermetic enclosureto compensate for the drift in time.

Thus referring to FIG. 1, there is shown a Wheatstone bridge 10. TheWheatstone bridge 10 typically includes piezoresistors 12, 13, 14 and15, which as seen are wired in the Wheatstone bridge configuration. TheWheatstone bridge has a bias supplied via a voltage regulator 16 whichis an analog device and which essentially provides a regulated voltageat its output which is used to bias the Wheatstone bridge. TheWheatstone is returned to the ground terminal through a span resistor 17as shown in FIG. 1. The configuration of such Wheatstone bridge is wellknown and, for example, reference is made to U.S. Pat. No. 5,614,678entitled High Pressure Piezoresistive Transducer issued to A. D. Kurtzet al. and assigned to Kulite Semiconductor Products, Inc. The entiredisclosure of U.S. Pat. No. 5,614,678 is hereby incorporated byreference.

In a Wheatstone bridge configuration 10, the piezoresistors measure thestress in a silicon diaphragm which is a direct function of the pressureapplied to the diaphragm. In the high temperature sensing device, theelectric connections between the sensor elements and the packagingheader are established via wire bonds. Alternatively, the leadless SOItechnology disclosed in U.S. Pat. No. 5,955,771 entitled Sensor for Usein High Vibrational Applications and Methods of Fabricating the Sameissued to A. D. Kurtz, et al. uses the same fabrication techniques withthe sensing chip packaged in a way that eliminates the use of wirebonds. In this SOI leadless connection, the leadless sensor is mounteddirectly on top of the header with the header pins protruding into thecontact regions. This is well known and is also described in the aboveU.S. Pat. No. 7,231,828.

In any event, as seen the voltage regulator is part of an analoginterface which includes an analog amplifier 18. The amplifier 18 is anoperational amplifier and is typically fabricated utilizing SOItechniques. In any event, both the sensor, the voltage regulator and theamplifier are fabricated using SOI technology allowing for hightemperature operation. It is also understood that if the analoginterface is remote from the SOI chip, these can be fabricated usingother techniques. In any event, the output of amplifier 18 is coupled tothe input of an analog to digital converter 19. The analog to digitalconverter 19, as is well known, can be implemented by manyconfigurations and essentially the analog to digital converter 19converts the analog output or the amplified bridge output from amplifier18 into a digital signal at output 25 which is applied to a real timeinput of the microprocessor 30. There is also an analog to digitalcircuit 20 which receives an output from the span resistor 17 which isapplied to an input of the microprocessor 30 via the output lead 26. Asone can see, one can access the microprocessor by means of temperatureor provide the microprocessor with a signal proportional to temperaturevia ADC 20.

Reference is now made to U.S. Pat. No. 4,192,005 issued on Mar. 4, 1980entitled Compensated Pressure Transducer Employing Digital ProcessingTechniques by A. D. Kurtz and assigned to Kulite Semiconductor Products,Inc. That patent shows a semiconductor sensor which includes amicroprocessor where compensation of the sensor is provided by themicroprocessor or digital processing circuit which accesses a memory atdesired locations to retrieve stored values and to process these valuesin order to compensate the output signal supplied by the bridge duringoperation. The system provides a compensated output signal trulydeterminative of the applied pressure as being compensated for theparticular sensor. Thus the above noted patent is also incorporatedherein in its entirety.

The microprocessor 30 can allow digital compensation of the transducer10 thereby achieving a much better accuracy than using simple analogmeans. As seen, a separate memory circuit 31 allows the storage of otheruseful information such as the part number of the transducer, the serialnumber, the last calibration date as well as capability ofre-calibration of the transducer without opening its hermetic enclosureas indicated above. The digital circuit, such as the analog to digitalconverters 19 and 20, the microprocessor 30 and the memory 31 are allwell suited for implementation using SOI technology. In fact, thedigital circuits, while more complex in terms of the number oftransistors employed as compared to analog circuits, are easier tomanufacture. Thus if one refers to the above-noted patent U.S. Pat. No.7,231,828 the analog circuits utilized therein are more complex and moredifficult to manufacture than the digital circuits described herein.This is due to the fact that in the operation of a digital circuit,there is no need for very well matched transistors as those with verytight control characteristics. Thus any transistor practically will workin a gate type circuit which output, of course, only has two states suchas a high state and a low state. Also digital circuits are much moretolerant of leakage currents, which can significantly affect an analogcircuit.

The analog to digital converter 19 (ADC) reads the analog output of thesensor via amplifier 18 in a digital format. As indicated the ADC can beimplemented in many ways: successive approximation register (SAR),sigma-delta, dual slope, as well as other techniques. All suchtechniques for implementing analog to digital converters have been usedsuccessfully with standard CMOS circuits, and can be similarlyimplemented using CMOS SOI technology. The microprocessor 30 receivesthe output from the analog to digital converter 19 via line 25 which maybe a serial communication bus. The microprocessor performs arithmeticand logic calculations to compensate the output of the sensor fortemperature effects. This as indicated has been described in U.S. Pat.No. 4,192,005. It also reads other data stored in the memory 31 asindicated above which would be the transducer part number, serialnumber, last calibration date and communicates this data to othersystems using the serial output 35.

The implementation of a general purpose microprocessor 30 is difficultusing the SOI process. In any event, a specialized reduced instructionset microprocessor is employed with most of the difficulties eliminated.Thus a separate memory 31 is used by the microprocessor to store theprogram and operate the microprocessor 30 in regard to data specific tothe sensor array 10. The microprocessor may also contain a small memorydesigned for operation employed with a sensor 10. The analog to digitalconverter 20, as indicated, converts the voltage output at the spanresistor to a digital output on bus 26. This digital output is appliedto the microprocessor and is indicative of temperature. As thetemperature varies, the voltage across the span resistor will vary andthe microprocessor using the variation in temperature can compensate thesensor 10. The circuits as described above, namely the microprocessor30, the ADC converters 19 and 20, and the memory, are shown as discretedevices, as the present state of the art makes it easier to design,manufacture and test them separately. These circuits can be implementedinto a single integrated circuit chip without significant technical lossand cost advantages.

Referring to FIG. 2 there is shown a block diagram of the memory 31which as indicated in FIG. 1 is coupled to the microprocessor 30. In anyevent, as indicated above, the memory 31 stores in its memory the partnumber as the sensor part number as well as the transducer part numberin memory location 40. The serial number is stored in memory location 41while the calibration date is stored in memory location 42. In memorylocation 44 the microprocessor program is stored thus enabling themicroprocessor to be of a simpler design. The memory 31 can store inmemory portions of the microprocessor program or the entiremicroprocessor program. The memory 31 may also store in memory amicroprocessor test routine in memory location 45. The microprocessortest routine, as can be seen from FIG. 1, will enable data from thememory stored in memory location 45 to be applied to the microprocessor30. The microprocessor 30 then can communicate with the digital outputbus 35. During this condition, the memory 31 will supply a predeterminedtemperature signal to the microprocessor as well as a predeterminedvoltage outputs to the microprocessor. These two signals would beindicative of a known temperature as well as a known output at thattemperature. Therefore, an anticipated value or digital output from themicroprocessor is determined. This value of course is known as thesystem has been calibrated accordingly. Thus when the proper pressuredata and temperature data is sent to the microprocessor during the testmode, the microprocessor then will produce an output at the bus 35,which output is predetermined and if not corresponding to thepredetermined output than a test indication signal will be providedindicating that there is something wrong with the microprocessor.

In a similar manner, a system test can be implemented by storage of thesystem process in memory location 46. One type of system test would beimplemented by the microprocessor 30. The memory 31 would indicate tothe microprocessor that a system test is to be performed by accessingthe location of system test parameters 46. The microprocessor will theninactivate the voltage regulator 16 and apply a digital output signal tothe digital to analog converter 36. The digital to analog converter 36will produce, at its output, an analog or voltage signal according tothe digital signal supplied by the microprocessor. This signal wouldprovide a bias to the bridge, which bias to the bridge would cause thebridge to produce a different output. The output would be predeterminedbased upon previous measurements, would be applied to amplifier 18 andhence to the analog to digital converter 19. If the output is correct,the microprocessor will produce the correct output on the bus 35. Themicroprocessor may produce various DC signals indicative of variousvoltages which would be applied to the bridge, whereby the bridge outputwould be known for these voltages as previously measured. In thismanner, if there is any drastic changes in the sensor values or resistorvalues, this can be determined during system tests. There is, of course,other techniques for testing the system and this is merely one by way ofexample. Hence it is seen that the use of the above-noted componentsenables one to provide significant capability to the operation of thetransducer while maintaining the capability of operation at hightemperatures because of the SOI technology. It should therefore beapparent to one skilled in the art that many alternative embodiments canbe envisioned. All of which are deemed to be part of the presentinvention and encompassed by the scope of the claims appended hereto.

1. A high temperature pressure sensing system, comprising: a pressuresensing sensor that outputs an analog signal indicative of a sensedpressure condition; an analog to digital converter that receives theanalog signal, converts the analog signal to a digital signal, andoutputs the digital signal; and a microprocessor that receives thedigital signal, performs calculations to compensate for temperatureeffects, and produces a digital output indicative of the sensed pressurecondition; the pressure sensing system being adapted to operate inenvironments exceeding 175° C.
 2. The high temperature pressure sensingsystem of claim 1, further comprising memory coupled to themicroprocessor, wherein the memory is operative to provide at least aportion of a control program for the microprocessor.
 3. The hightemperature pressure sensing system of claim 2, wherein the memorystores data specific to the pressure sensing system.
 4. The hightemperature pressure sensing system of claim 2, wherein the memorystores a microprocessor test routine.
 5. The high temperature pressuresensing system of claim 2, wherein the memory stores a sensing systemtest routine.
 6. The high temperature pressure sensing system of claim1, wherein the pressure sensing senor is a Wheatstone bridge comprisingfour piezoresistors.
 7. The high temperature pressure sensing system ofclaim 6, further comprising an analog voltage regulator coupled to theWheatstone bridge for biasing the Wheatstone bridge.
 8. A hightemperature pressure sensing system, comprising: a Wheatstone bridgethat outputs a first analog signal indicative of a sensed pressurecondition; an amplifier that receives the first analog signal andoutputs an amplified first analog signal; a first analog to digitalconverter that receives the amplified first analog signal, converts theamplified first analog signal to a first digital signal, and outputs thefirst digital signal; a span resistor coupled in series with theWheatstone bridge that outputs a second analog signal indicative of asensed temperature condition; a second analog to digital converter thatreceives the second analog signal, converts the second analog signal toa second digital signal, and outputs the second digital signal; and amicroprocessor that receives the first and second digital signals,performs calculations to compensate for the sensed temperaturecondition, and produces a digital output indicative of the sensedpressure condition; the pressure sensing system being adapted to operatein environments exceeding 175° C.
 9. The high temperature pressuresensing system of claim 8, wherein the Wheatstone bridge is fabricatedusing SOI techniques.
 10. The high temperature pressure sensing systemof claim 8, wherein the first and second analog to digital convertersare fabricated using SOI techniques.
 11. The high temperature pressuresensing system of claim 8, wherein the microprocessor is fabricatedusing SOI techniques.
 12. The high temperatures sensing system of claim8, wherein the amplifier is fabricated using SOI techniques.
 13. Thehigh temperature pressure sensing system of claim 8, further comprisingmemory coupled to the microprocessor and operative to provide at least aportion of a control program for the microprocessor.
 14. The hightemperature pressure sensing system of claim 13, wherein the memorystores data specific to the pressure sensing system.
 15. The hightemperature pressure sensing system of claim 13, wherein the memorystores a microprocessor test routine.
 16. The high temperature pressuresensing system of claim 13, wherein the memory stores a sensing systemtest routine.
 17. The high temperature pressure sensing system of claim1, wherein the pressure sensing system is adapted to operate inenvironments exceeding 250° C.
 18. The high temperature pressure sensingsystem of claim 1, wherein the pressure sensing system is adapted tooperate in environments up to 300° C.
 19. The high temperature pressuresensing system of claim 8, wherein the pressure sensing system isadapted to operate in environments exceeding 250° C.
 20. The hightemperature pressure sensing system of claim 8, wherein the pressuresensing system is adapted to operate in environments up to 300° C.